It is presently required that an operator climb up on the roof of a hopper car to open a hatch before the car is unloaded. This is required to prevent a vacuum from developing in the car as the lading is withdrawn through an outlet in the bottom of the car. Occasionally an operator will forget to open a hatch before unloading a car and vacuum suction unloading equipment will cause severe structural damage to the car. Climbing on the car to open a hatch is an inconvenience to the operator and is dangerous in the winter months when the car's ladders and walkways are coated with snow and ice.
In U.S. Pat. No. 1,578,766 a vacuum relief valve is disclosed in which a valve member is vertically movable from a closed, seated position to an open, unseated position which provides fluid communication between first passageways extending from the outside atmosphere to the lower surface of the valve, and second passageways providing fluid communication between the interior of the tank and the upper surface of the valve member. When the pressure of the outside atmosphere exceeds the pressure in the tank the valve member is lifted to equalize the pressure. However, the cross sectional area of the first and second passageways are both smaller than the cross sectional area of the valve and not sufficiently large as to maximize the volume of air flow for a given pressure drop and valve cross sectional area.
U.S. Pat. Nos. 1,897,076 and 2,732,856 disclose vacuum valves in which the cross sectional area of first passageways in communication with the outside atmosphere is equal to the cross sectional area of the valve. However, the second passageways providing fluid communication between the valve and the container are laterally spaced from the valve. Thus a large cross sectional area is required to accommodate the valve and the second passageways. This would not be compatible with the limited clearance available on top of a railway freight car.
In U.S. Pat. No. 2,169,410, a combination pressure relief valve and vacuum relief valve is disclosed in which only a portion of the cross-sectional area of the pressure relief valve is used as a vacuum relief valve. Thus the volume of air flow into the container is not maximized for a given pressure drop and available valve area.